The mechanics of adherent sheets is central to applications ranging from patching a band aid, coating technology, to the breakthrough discovery of peeling graphene flakes using sticky tape. These processes are often hindered by the formation of blisters and loops, which are notoriously difficult to remove. Here we describe and explain a remarkable phenomenon that arises when one attempts to remove a loop in a self-adherent sheet that is formed by, e.g., folding two adhesive sides of a tape together. One would expect the loop to simply unloop when pulling on its free ends. Surprisingly, however, the loop does not immediately open up but shrinks in size, held together by a tenuous contact region that propagates along the tape. This adhesive contact region only ruptures once the loop is reduced to a critical size. We experimentally show that the loop-shrinkage results from an interaction between the peeling front and the loop, across the contact zone. This new type of interaction falls outside the realm of the classical elastica theory and is responsible for a highly nonlinear increase in the peeling force. Our results reveal and quantify the increased force required to remove loops in self-adherent media, which is of importance for blister removal and exfoliation of graphene sheets.

Reciprocity between cells and their surrounding extracellular matrix is one of the main drivers for cellular function and, in turn, matrix maintenance and remodelling. Unravelling how cells respond to their environment is key in understanding mechanisms of health and disease. In all these examples, matrix anisotropy is an important element, since it can alter the cell shape and fate. In this work, the objective is to develop and exploit easy-to-produce platforms that can be used to study the cellular response to natural proteins assembled into diverse topographical cues. We demonstrate a robust and simple approach to form collagen substrates with different topographies by evaporating droplets of a collagen solution. Upon evaporation of the collagen solution, a stain of collagen is left behind, composed of three regions with a distinct pattern: an isotropic region, a concentric ring pattern, and a radially oriented region. The formation and size of these regions can be controlled by the evaporation rate of the droplet and initial collagen concentration. The patterns form topographical cues inducing a pattern-specific cell (tenocyte) morphology, density, and proliferation. Rapid and cost-effective production of different self-agglomerated collagen topographies and their interfaces enables further study of the cell shape-phenotype relationship in vitro. Substrate topography and in analogy tissue architecture remains a cue that can and will be used to steer and understand cell function in vitro, which in turn can be applied in vivo, e.g. in optimizing tissue engineering applications.

Photogrammetry uses images of a three-dimensional structure to derive information about its shape and position. In this work, a photogrammetric technique is implemented with a single camera and a digital projector to measure changes in an underwater sediment bed. This implementation incorporates refraction at an interface allowing for measurements through a deformed or changing water surface. The digital projector provides flexibility in choosing projected patterns and has a high frame rate, which allows to easily increase the spatial and temporal resolution of the measurements. The technique requires first for both the camera and the projector to be calibrated using triangulation. With the calibration, we construct lines in three-dimensional space that originate from the projector and the camera, and intersect on the surface to be measured. To correctly incorporate refraction due to a change of medium, each line in space is recalculated from its intersection with the interface using Snell's law. This has the benefit that only one calibration for measurements is needed if the location and shape of the interface are known. The technique is validated by measuring a submerged undulated surface, plastic objects and a sediment bed. In particular, the undulated plate is reconstructed under a flat and a parabolic water surface. Finally, the technique is used in combination with particle image velocimetry to dynamically measure a changing sediment bed under an oscillating flow and the flow velocity at the free surface.